1. Field Of The Invention
The present invention relates to analog integrated circuits. More particularly, the present invention relates to MOS and MOS/bipolar amplifier circuits.
2. The Prior Art
It has recently become apparent that large scale analog circuits can be achieved using conventional CMOS technology. The key to achieving very high levels of complexity in an analog system is to operate the individual transistors in their subthreshold region, where the drain current is an exponential function of the gate-source voltage. In this regime of operation, amplifiers can be operated with current levels in the range from 10.sup.-12 A to 10 .sup.-7 A. At these low currents, the drain current of the individual transistors saturates at drain voltages above 100 to 200 Mv, allowing analog operation with the same power-supply voltages commonly employed for digital circuits (0-5 V in 1988). Because of the low power-supply voltage and low current level, the total power dissipated by an individual amplifier is extremely small, making possible large-scale systems employing 10.sup.4 or more amplifiers.
Despite the numerous advantages of subthreshold operation, very few systems outside of the electronic watch industry have taken advantage of this mode of operation. The major problems that have prevented application of subthreshold amplifiers have been their input offset voltage and the limited input voltage range.
Previous circuits are known for converting an input voltage to an output current. Such a circuit, shown in FIG. 1a, is described in the book Analog VLSI and Neural Systems by Carver Mead, Addison-Wesely Publishing Co. 1989, at page 70. The output current of the circuit of FIG. 1a is plotted as a function of the differential input voltage in FIG. 1b. As may be seen from FIG. 1b the current is a linear function of the differential input voltage over only a small portion (about 100 mV) of the input range. The advantages of a circuit which can produce an output current which is a linear function of its input voltage over a wider range of input voltages are obvious.
One prior art approach to providing such a circuit is described in U.S. Pat. No. 4,935,702 to Mead et al., assigned to the same assignee as the present invention. This patent discloses a circuit which utilizes a capacitive voltage divider to extend the range over which the output current is a linear function of the input voltage. The approach to extending input voltage range used in this prior invention limited the signal excursion seen by the input transistors of the amplifier to the approximately 100 mV allowed by the technology. However, by reducing the input voltage excursion, the signal-to-noise ratio of the amplifier is inevitably degraded.
It is thus an object of the present invention to provide a circuit whose output current is a linear function of its input voltage over a major portion of the power supply range without degrading the signal-to-noise ratio.